Postmenopausal osteoporosis results from defective bone remodeling stemming from estrogen (E2) deficiency. We have recently reported that bone loss during E2 deficiency is driven principally by upregulated T cell-derived cytokine secretion including the key osteoclastogenic cytokines, Receptor-activator of NFkB Ligand (RANKL) and Tumor necrosis Factor alpha (TNF). A key event in inducing T cell activation and cytokine secretion is the upregulation of IL-7 production, a consequence of estrogen deficiency. We have recently reported that inhibiting the elevated levels of IL-7 induced by ovariectomy (ovx) in mice using a neutralizing antibody, is efficacious in preventing ovx-induced bone loss in mice in vivo. This suggests that IL-7 neutralization may be a novel means to prevent and treat osteoporosis. However, despite the success of this experiment, long-term therapeutic treatment with antibodies in humans often lead to serious clinical complications and poor long-term efficacy. In order to overcome the limitations of antibody-based therapies, we now propose a novel strategy to neutralize IL-7 in vivo and ameliorate bone loss in postmenopausal osteoporosis. This strategy involves the design of a novel peptide based therapeutic agent comprising the soluble extracellular ligand-binding domain of the mouse IL-7 receptor alpha (slL-7Ralpha). To extend in vivo half-life this peptide will be expressed as a fusion protein with the human IgG1-Fc region (slL-7Ralpha-Fc). We hypothesize that this agent will function in vivo as a soluble IL-7 decoy receptor and prevent bone loss during E2 deficiency, by neutralizing excess IL-7. We propose two Specific Aims to further investigate our hypothesis: Specific Aim 1: To demonstrate that a soluble IL-7 decoy receptor is an efficacious therapeutic adjunct in preventing estrogen deficiency-induced bone loss in vivo. This will be achieved by injecting purified recombinant slL-7Ralpha-Fc into mice following ovx and examining physical, biochemical and histomorphometric indices of bone turnover following ovariectomy. Long-term stability and biological half-life of slL-7Ralpha-Fc will be evaluated by measuring levels of free IL-7 and slL-7Ralpha-Fc concentrations in blood serum as a function of time. Specific Aim 2: To demonstrate the feasibility of preserving bone mass following estrogen deficiency by expressing a soluble IL-7 decoy receptor in vivo, using a gene transfer approach. This Specific Aim will be achieved by the introduction into ovx mice of a mammalian expression vector encoding slL-7Ralpha-Fc by application of two different gene transfer procedures, intramuscular injection targeting skeletal muscle and hydrodynamic injection targeting the liver. These Specific Aims will demonstrate two important "Proofs of Principle": (1) a novel therapeutic agent slL-7Ralpha-Fc, is an efficacious therapy for the prevention of bone loss following estrogen deficiency. (2) Gene transfer is an effective mechanism for introducing and expressing slL-7Ralpha-Fc in vivo.